Inspection and metrology technologies are conventionally used in semiconductor fabrication facilities for material monitoring, disposition, yield prediction, and yield management. Wafers are inspected at various stages of production using both in-line and off-line processes. Cell-to-cell inspection is a mode wherein locally repeating structures are compared to each other, and any noted difference is declared to be a defect. Cell-to-cell inspection is used in both wafer and reticle inspection. This modality has advantages in that the reference data is very closely spaced to the test region so that the inspection tool does not need to be particularly stable to successfully employ this approach.
Recent changes in lithographic approaches make cell-to-cell inspection more problematic. Model-based Optical-Proximity-Correction (OPC) for optical masks and Flare Correction for extreme ultraviolet (EUV) masks can lead to very subtle differences in the design of nearly repeating patterns. These changes may include, for example, a very small jog in a long straight line with no apparent purpose. Existing methods of determining whether a region is sufficiently repeating for the application of a cell-to-cell detector uses the images themselves. This means that a subtle design difference between cells can easily be declared to be a defect rather than an intentional design feature. It is this false defect mechanism that can limit the sensitivity and applicability of the cell-to-cell defect detectors.
Consequently, it would be advantageous if an apparatus existed that is suitable for identifying cells in a reticle appropriate for cell-to-cell inspection.